There is proposed a semiconductor storage device such as NAND flash memory in which floating gates at a memory cell transistor include a lower floating gate, an upper floating gate, and an intergate insulating film provided between the lower floating gate and the upper floating gate. The charge holding property can be enhanced due to such a structure.
However, in the semiconductor storage device with the above-described structure, the intergate insulating film provided between the lower floating gate and the upper floating gate is so thick that transmission of charges due to FN (Fowler-Nordheim) tunneling is dominant, and thus there is a problem that a tunnel current cannot be increased and the write property or erase property is deteriorated.